Impact copolymers having improved properties

ABSTRACT

Improvements in the aesthetic appearance and performance properties of heterophasic polymers is obtained through the breaking up and dispersion of large gels. According to the current invention, a novel process is provided for filtration of heterophasic polymers using a fiber metal felt (FMF) media. Molded articles made from impact copolymers prepared according to the present invention have improved appearance and fracture mechanics relative to impact copolymers produced according to prior art methods.

FIELD OF THE INVENTION

The present invention relates generally to polypropylene impactcopolymers. More particularly, the present invention relates toimproving the properties of polypropylene impact copolymers by improvingthe dispersion of gels within the impact copolymers.

BACKGROUND OF THE INVENTION

It is common industrial practice to use standard mesh screens, such aswoven metal plain weave or dutch twill, to filter a polymer melt duringextrusion to remove dirt and foreign matter, and in the case of somepolymers to remove polymeric gels. Gels are of particular concern in thecase of heterophasic polymer blends, such as polyolefin impactcopolymers, since the presence of large numbers of large gels maycompromise the aesthetic appearance of the copolymers and may adverselyaffect the performance of the copolymers. U.S. Pat. No. 5,730,885teaches a method for reducing the number and size of polymeric gels in apolypropylene blend by filtering the blend through multiple filterscreens (screen packs) during extrusion of the blend. However, standardwire screens are subject to deformation and failure at the highpressures required for polymer filtration, which limits theirefficiency. U.S. Pat. No. 3,197,533 purports to address this problemthrough the use of a microporous sintered metal plate for reducing gelsin various polymers to micron size. However, this patent teaches thatminimum pressures of 5500 psi across the sintered metal plate arerequired. U.S. Pat. No. 4,126,560 discloses a filter media comprisingsintered metal fibers for removing gels from molten polymers. The filtermedia is described as being for use in conjunction with a fiber spinningprocess wherein gels need to be removed to prevent clogging of thespinneret and/or fiber breakage. It is specifically disclosed that gelsof progressively smaller sizes are trapped in the filter media, whichhas progressively smaller pore openings. As disclosed in “Primer onMetal Filtration Media”, E. Gregor & Associates, LLC, © 2003, such fibermetal felt media has been used in the synthetic textile industry forfiltering polyester and nylon.

However, in the case of polyolefin impact copolymers complete removal ofpolymeric gels is undesirable since these gels contribute to the impactresistant properties of the impact copolymer.

It is therefore desirable to provide a process that improves theaesthetic appearance and performance of polyolefin impact copolymers byeliminating large polymeric gels while maintaining the presence of andimproving the dispersion of smaller gels that contribute to theperformance properties of polymer.

SUMMARY OF THE INVENTION

The present invention provides a process for reducing gel size andimproving gel dispersion in a polypropylene polymer. The processcomprises extruding a polymer melt comprising a heterophasic blend of(a) polypropylene homopolymer or random copolymer as a continuous phase,and (b) a polypropylene copolymer rubber as a dispersed phase, andpassing the polymer melt through at least one layer of a filter mediacomprising a fiber metal felt layer. After passing through the filtermedia, the polymer melt may be quenched and pelletized. Alternatively,the melt may be used directly to produce an injection molded part, orpassed through a die to form a shape that may be stretched into a filmor other shape, such as a bottle. The polymer melt preferably comprisesa propylene-ethylene impact copolymer wherein the continuous phasecomprises either a polypropylene homopolymer or random copolymer withethylene, and the dispersed phase is a propylene-ethylene rubber.

The fiber metal felt (FMF) media useful in the process of the presentinvention comprises a layer of non-woven metal fibers. Preferably thefibers are a stainless steel construction, but may be fabricated fromother metals.

According to one preferred embodiment of the invention, the FMF media isbacked by at least one wire mesh screen, which may be of any mesh size,but is preferably in the range of 20 to 325 mesh. The wire mesh screenmay be diffusion bonded to the FMF or may be separate. In anotherpreferred embodiment, the polymer melt is passed through at least twolayers of FMF media, which may be separated by one or more standard wiremesh screens.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides polypropylene impact copolymers havingimproved properties as a result of reduced gel size and improved geldispersion. The process for obtaining the improved polypropylene impactcopolymers comprises passing a polymer melt of a heterophasic copolymerthrough a least one layer of a filter media comprising a fiber metalfelt (FMF) layer so that large gels are broken up and uniformlydispersed throughout the continuous phase.

The process according to the current invention is useful to improve theaesthetic appearance and properties of any heterophasic copolymer, butis particularly useful in improving the aesthetic appearance andproperties of impact copolymers comprising a blend of a polypropylenehomopolymer or random copolymer as a continuous phase and a dispersedphase comprising a propylene-ethylene or other propylene-α-olefincopolymer rubber. As used herein, “blend” refers to either an in-reactorproduced blend, or a physical blend of two or more separately producedpolymers. Impact copolymers with a high content of rubber areparticularly susceptible to the presence of gels, which compriseagglomerates of propylene-ethylene or other propylene-α-olefin copolymerrubber. The presence of gels plays a role in the impact resistantproperties displayed by impact copolymers. However, the presence oflarge numbers of large gels can adversely affect the performanceproperties of the polymer, as well as the aesthetic appearance of thepolymer in applications such as films and for automobiles. Polypropyleneimpact copolymers produced according to the current invention displayimproved aesthetic appearance as manifested reduced appearance of largegels, as well as improved ductile behavior as manifested by improvedfracture mechanics, including enhanced shear yielding and crazing, andreduced bifurcation, relative to impact copolymers produced usingtraditional mesh screens.

The process according to the present invention alleviates the problem oflarge numbers of large gels by breaking up and dispersing large gelspresent in reactor powder or vis-broken impact copolymers. The processaccording to the current invention achieves at least a 50 percentreduction in gels larger than 550 μ. Preferably, the process of thecurrent invention achieves a 70 to 80 percent reduction in gels largerthan 550 μ. The breaking apart and dispersing of large gels is not to beequated with the retention or removal of gels, as taught in priorreferences. Although some level of gel retention is unavoidable in anyfiltering process, gel retention is preferably kept to a minimum in thepractice of the present invention. As stated above, the presence ofdispersed gels contributes to the impact properties of impactcopolymers. Therefore, retention of significant amounts of gels on thefilter media, leading to their removal from the polymer may adverselyaffect these properties of the polymer. In addition, such retention ofgels leads to the ultimate clogging of the filter media and impairmentof its effectiveness, requiring more frequent changes of the media.

The FMF media used as filter media in the process according to thepresent invention comprises a non-woven layer of metal fibers having astable porosity. FMF media of this type are described in “PolymerFiltration Media Selection”, R. Geary and J. Litschert, Chemical FibersInternational, 48, September 1998, pp. 328, 30, 32. FMF media of variouscoarseness are useful in the present invention, however, since the metalfibers are arrayed in a random non-woven pattern it is not appropriateto refer to a given media as having a single porosity, as with standardmesh screens. In fact, each layer of FMF media may have a gradient poresize design, wherein a melt passing through the media encounters a pathof decreasing diameter. The preferred FMF media for use in the presentinvention are equivalent to standard wire mesh screens rated atporosities of from 1 to 150 microns, preferably 5 to 100 microns, morepreferably 60 to 100 microns. Specific examples include FMF media ratedat a nominal porosity of 60, 75, 100 and 150 microns. Nominal porosityis defined as the smallest particulate that a particular FMF media iscapable of retaining. In the process according to the current inventionthe FMF media may be supported by a standard mesh screen. The supportscreen may be either diffusion bonded to the FMF media or separate. Ingeneral the mesh support screen has a low mesh size, i.e. high porosity,and is not believed to significantly improve the dispersion of gels overthat obtained by the FMF media and is not necessary to the functioningof the invention, and is therefore optional. However, it is contemplatedthat additional standard screens of sufficiently high mesh may be usedin conjunction with the FMF media to improve gels dispersion. Accordingto an alternative embodiment of the invention, more than one layer ofthe FMF media may be used. Use of two or more layers of FMF mediaproduces a further improvement in gel dispersion. In an embodiment wheretwo or more layers of FMF media are used the layers may be separated bya mesh screen, which may in some instances contribute to the dispersionof gels, depending on the mesh of the screen. In addition, mesh supportscreens may also be used with the combination of two or more FMF layers.

A series of trials were run to compare the process according to thepresent invention to prior art methods using standard wire mesh screens.All examples were run using either a 60 or 110 melt flow ratepropylene-ethylene impact copolymer powder comprising a polypropylenehomopolymer continuous phase and a dispersed phase of propylene-ethylenerubber, with an additive package that included antioxidants, acidscavengers and nucleating agents. Melt flow rate (MFR) was measuredusing ASTM method D-1238 at 230° C. using a 2.16 kg load.

EXAMPLE 1 Fiber Metal Felt Media 60 Micron

A 60 MFR impact copolymer reactor powder was extruded and passed througha single layer of FMF media having a 1 inch cross section and a nominalrated porosity of 60 microns; part no. 60 AL3, available fromPUROLATOR®. The powder was pelletized using a 0.75 inch single screwextruder with a maximum screw speed of 80 rpm, which was equipped withsingle strand die plate and a melt pump. There were four heating zones.The pressure maximum was 3000 psi. The extruder temperature conditionswere set as follows: zone 1: 450° F.; zone 2: 475° F.; melt pump: 475°F.; and die plate 475° F. The screw speed was set at 68 and 40 rpm. Themelt pump rpm was adjusted to control set screw speed.

EXAMPLE 2 Fiber Metal Felt Media 75 Micron

A 60 MFR impact copolymer reactor powder was extruded and passed througha single layer of FMF media having a 1 inch cross section and a nominalrated porosity of 75 microns; part no. 75 AL3, available fromPUROLATOR®. The extruder temperature conditions, screw speed and pumpspeed were set as in Example 1.

COMPARATIVE EXAMPLE 3 Wire Mesh Screens

A 60 MFR impact copolymer reactor powder was extruded and passed througha screen pack comprising four wire mesh screens having a 1 inch crosssection in a 20/40/200/40 mesh alignment. In this alignment the 200 meshscreen is responsible for the elimination of large gels. The extrudertemperature conditions, screw speed and pump speed were set as inExample 1.

EXAMPLE 4 Dual 60 Micron Fiber Metal Felt Media

The procedure of Example 1 was repeated, except that two layers of FMF,each having a nominal rated porosity of 60 microns, were used.

EXAMPLE 5 Dual 60/75 Micron Fiber Metal Felt Media

Example 5 using two layers of FMF media was repeated, except that afirst FMF layer having a nominal rated porosity of 60 microns was backedwith a second FMF layer having a nominal rated porosity of 75 microns.

The pelletized polymer from Examples 1-5 was then used to produce filmsfor visual evaluation and testing using a Haake mini-extruder. The filmsproduced from Examples 1-5 were analyzed using line scanning digitalcamera imaging installed on the film line (Southern Analytical). Thefilms produced from the pellets that were passed through the FMF mediadisplayed a much smaller number of large gels, and further displayed alarger number of well dispersed small gels.

Five inch flex/DTUL (deflection temperature under load) specimens (ASTMD790, D648), were produced from the pellets from Examples 1-5 to testfracture mechanics using the double notch 4-point charpy impact test(DN-4PB) and for DN-4PB slow crack propagation experiments. Thesetechniques are described in “Study of fracture mechanisms of multiphasepolymers using the double-notch four-point bending method”, H.-J. Sue etal., J. Materials Sci., 28 (1993) 2975-2980. The DN-4PB test is aneffective technique to examine the toughening mechanisms, e.g. shearyielding, crack bifurcation, crazing and path deflection, of polymersand determine the sequence of the various toughening events observed inthe damage zone of the fracture.

The DN-4PB charpy impact test at room temperature yielded similarquantitative results for the FMF media and for the 200 mesh screencontrol as shown in Table 1. This indicates that there was nodetrimental effect on charpy impact from using FMF media.

TABLE 1 DN-4PB Impact Quantitative Results. Example 3: 200 mesh Example1: 60μ FMF Example 2: 75μ FMF screen (control) Average: 0.72 ft. lb./Average: 0.72 ft. lb./ Average: 0.70 ft. lb./in² in² in² Std. Dev.: 0.04Std. Dev.: 0.02 Std. Dev.: 0.03 RSD %: 5.2 RSD %: 2.3 RSD %: 4.3

The samples produced using the FMF media in Examples 1 and 2 weredifferentiated from the control produced using a 200 mesh screen inExample 3 when observed using optical microscopy under cross polar lightmode. The fracture mechanics of the samples, determined by the DN-4PBslow crack propagation experiments show that the samples produced inExample 3 exhibited more bifurcation relative to Examples 1 and 2.Further, Examples 1 and 2 displayed a greater degree of crazing, whichis preferred. The greater degree of bifurcation in the samples producedusing the 200 mesh screen is thought to be due to the presence of higheramounts of large gels than in the samples produced using the FMF media.

In addition, despite the low rated porosity of the FMF media, the systempressures experienced during extrusion and filtration steps were wellwithin the 3000 psi maximum limit. The 75 AL3 FMF media displayed a diepressure plateau at about 1000 psi. The 60 AL3 FMF media displayed a diepressure plateau at about 1750 psi. This indicates that screen bulgingand catastrophic screen failure associated with excessive pressure isless of a concern with FMF media.

Gel count results for Examples 1-5 are displayed in Table 2. The resultsin Table 2 demonstrate that the use of the FMF material substantiallyreduced the number of large gels relative to the standard 200 mesh wirescreen. Notably the gel count for gels averaging 550μ and higher for thesamples produced using the 60 and 75 micron FMF media is significantlylower than for the standard 200 mesh screen. In addition, the totalnumber of gels averaging 250μ and higher is substantially lower in bothsamples produced using the FMF media. The use of two layers of FMF mediaresulted in a reduction in gels averaging 550μ and larger and averaging250μ and larger which is substantially greater than 200 mesh screen.Gels averaging 800μ and larger were almost completely eliminated.

TABLE 2 Gel Count 100μ 200μ 300μ 400μ 500μ 600μ 700μ 800μ900μ >900μ >550μ >250μ Ex. ave. ave. ave. ave. ave. ave. ave. ave. ave.ave. ave. ave. Total 1 192472 115770 27773 5168 1148 306 101 37 13 8 46534554 342797 2 214928 146022 41804 8692 1987 494 123 45 11 10 683 53166414118 3 195767 151943 66248 23391 7990 2477 682 194 53 26 3432 101061448771 4 140467 68119 11937 1875 454 138 32 4 1 0 175 14441 223029 5166048 85736 14993 1893 304 65 12 4 1 1 83 17273 269057

Table 3 presents the same results as a percentage of total gels forExamples 1 through 5, using the 60 MFR polymer. The single FMF layersamples, 1 and 2 show only about 0.14 and 0.17 percent respectively ofgels having an average size of greater than 550μ, as compared toapproximately 0.76 percent for the sample produced using the 200 meshscreen in Example 3. There is also a significant difference for gelshaving an average size of 250μ or more. Samples 4 and 5, produced usingtwo FMF layers show less than 0.1 percent of gels having an average sizeof greater than 550μ.

TABLE 3 Gel Fraction 100μ 200μ 300μ 400μ 500μ 600μ 700μ 800μ900μ >900μ >550μ >250μ Ex. ave. ave. ave. ave. ave. ave. ave. ave. ave.ave. ave. ave. Total 1 56.15 33.77 8.10 1.51 0.33 0.09 0.03 0.01 0.0030.002 0.14 10.08 100.0 2 51.90 35.26 10.09 2.10 0.48 0.12 0.03 0.010.003 0.002 0.17 12.84 100.0 3 43.62 33.86 14.76 5.21 1.78 0.55 0.150.04 0.01 0.006 0.76 22.51 99.99 4 62.98 30.54 5.35 0.84 0.20 0.06 0.010.002 0.0005 0 0.07 6.46 100.0 5 61.71 31.87 5.57 0.70 0.11 0.02 0.0040.001 0.0004 0.0004 0.03 6.41 100.0

EXAMPLE 6 FMF Media 75 Micron

A pelletized impact copolymer was produced on a manufacturing scaleextruder by passing a polymer melt of a 110 MFR reactor impact copolymerpowder through a single layer of FMF media having a rated porosity of 75microns.

COMPARATIVE EXAMPLE 7 Wire Mesh Screen

A comparative example was run on the same production scale extruderequipment, using the same reactor impact copolymer powder as in Example6 to produce a pelletized polymer. The screen pack used in thecomparative trial had a 20/40/200/40 configuration.

Films using pellets from Examples 6 and 7 were produced for visualevaluation using a Haake mini-extruder. The films produced from Examples6 and 7 were analyzed using line scanning digital camera imaginginstalled on the film line (Southern Analytical).

Tables 4 and 5 show the gel count and percentage data for Examples 6 and7. In Examples 6 and 7 using the 110 MFR polymer, only about 0.5 percentof the gels in the sample produced using the 75μ FMF media have anaverage size of greater than 550μ, compared with about 2.3 percent ofthe gels in the sample produced using the standard 200 mesh screen. Inaddition, the data show that less than 11 percent of the gels in thesample produced using the FMF media average 250μ or higher, as comparedto over 19 percent for the samples produced using the standard meshscreen.

TABLE 4 Gel Count 100μ 200μ 300μ 400μ 500μ 600μ 700μ 800μ900μ >900μ >550μ >250μ Ex. ave. ave. ave. ave. ave. ave. ave. ave. ave.ave. ave. ave. Total 6 286144 142146 36003 9957 3249 1254 564 296 183195 2492 51701 479991 7 228149 125108 44330 19516 9779 5305 2666 1248494 341 10054 83679 436936

TABLE 5 Gel Fraction 100μ 200μ 300μ 400μ 500μ 600μ 700μ 800μ900μ >900μ >550μ >250μ Ex. ave. ave. ave. ave. ave. ave. ave. ave. ave.ave. ave. ave. Total 6 59.61 29.61 7.5 2.07 0.68 0.26 0.12 0.06 0.040.04 0.52 10.77 99.99 7 52.22 28.63 10.15 4.47 2.24 1.21 0.61 0.29 0.110.08 2.3 19.16 100.01

The invention has thus been explained with reference to specificexamples. However, the invention applies to a wide range of impactcopolymers from fractional (<1 g/10 min.) melt flow rates to 1500 g/10min. The invention may be particularly useful with impact copolymershaving melt flow rates ranging from 50 to 150 g/10 min. The process maybe used to reduce large gels and improve dispersion of gels in reactorpowder, as well as previously pelletized materials. It is alsocontemplated that the reactor powder or pelletized material may becompounded with one or more of several modifiers including, but notlimited to elastomers, rubber modifiers and oils. Further, the processis not limited to a single iteration of the filtration. In particular, areactor powder may be filtered through an FMF filter media according tothe current invention and pelletized. The pelletized polymer may then besubjected to a second melt extrusion step, wherein it is compounded withone or more of several modifiers including, but not limited toelastomers, rubber modifiers and oils. It is contemplated that thecompounded polymer melt may also be subjected to a second filtrationthrough an FMF media to reduce the occurrence of large gels and improvegel dispersion. The invention also applies to impact copolymerscontaining a wide variety of additives including, but not limited to,antioxidants, antistatics, nucleators and slip agents.

1. A process for improving gel size and distribution in a polypropylenepolymer comprising: extruding a polymer melt comprising a heterophasicblend of polypropylene homopolymer or random copolymer, and apolypropylene copolymer rubber; and passing the polymer melt through atleast one layer of a filter media comprising a fiber metal felt layer.2. The process according to claim 1, further comprising quenching andpelletizing the polymer melt.
 3. The process according to claim 1,further comprising injection molding the polymer melt to produce amolded article.
 4. The process according to claim 1, further comprisingpassing the polymer melt through a die to form an extruded shape; andstretching the extruded shape to form an article.
 5. The processaccording to claim 1, wherein the at least one layer of filter mediafurther comprises a mesh screen.
 6. The process according to claim 1,wherein the polymer melt is passed through at least two layers of filtermedia, each layer comprising a fiber metal felt.
 7. The processaccording to claim 1, further comprising, compounding the polymer meltwith at least one component selected from the group consisting ofelastomers, modifiers and oils prior to passing through the at least onelayer of filter media.
 8. The process according to claim 7, furthercomprising quenching and pelletizing the polymer melt.
 9. The processaccording to claim 7, further comprising injection molding the polymermelt to produce a molded article.
 10. The process according to claim 7,further comprising passing the polymer melt through a die to form anextruded shape; and stretching the extruded shape to form an article.11. The process according to claim 1, wherein the polymer melt comprisesa blend of a propylene homopolymer and a propylene-ethylene copolymerrubber.
 12. The process according to claim 1, wherein the polymer meltcomprises a blend of a propylene-ethylene random copolymer and apropylene-ethylene copolymer rubber.
 13. A pelletized polypropylenepolymer produced according to the process of claim 1, wherein about 12percent or less of polymeric gels in the pelletized polypropylenepolymer have an average size of 250 microns or more.
 14. A pelletizedpolypropylene polymer according to claim 13, wherein about 0.5 percentor less of polymeric gels in the pelletized polypropylene polymer havean average size of 550 microns or more.
 15. The process according toclaim 1, wherein the number of gels having a size of greater than 550microns is reduced by at least 50 percent.
 16. The process according toclaim 1, wherein the polymer melt is produced by melt extruding apelletized polymer.
 17. The process according to claim 7, wherein thepolymer melt is produced by melt extruding a pelletized polymer.